Once you understand the theory, it all comes right. Once you’ve finished the long flap for the back of your heel, you “turn” the heel by knitting/purling short rows [where you turn your work before reaching the true “end” of the row and start a new row] until all the stitches on the heel flap are used up.
Then you need to pick up new stitches along each side of your heel flap. These stitches will point towards the future toe of your sock, and be 90 degrees angle to the stitches on the heel flap. Like Jeremy and the Knitter say, the number of stitches you need to pick up along the side edges of the heel flap vary from pattern to pattern. This isn’t really a big deal - just so you can start decreasing and get the exact number of stitches needed to knit the rest of the sock foot till you start the toe decreases.
Hope this helps,
Dot